HumanizeBigNumber.cs

using System;
using System.Globalization;
using System.Linq;

public class Program
{
	/// <see cref="https://en.wikipedia.org/wiki/Metric_prefix#List_of_SI_prefixes" />
	
		public static void Main()
		{
			try
			{
				for (int i = 0; i < 27; i++) // see 43
				{
					var d = HumanizeBigNumber(Math.Pow(10, i));
					Console.WriteLine(d.PadRight(10) + DehumanizeBigNumber(d));
				}
			}
			catch (Exception ex)
			{
				Console.WriteLine(ex.Message);
			}

		}

		private static string HumanizeBigNumber(double bigNumber)
		{
			const string symbols = "kMGTPEZY";

			var numbers = bigNumber
				.ToString("0.##################E+0", CultureInfo.InvariantCulture) // scientific notation
				.Split(new[] { 'E', '+' }, StringSplitOptions.RemoveEmptyEntries) // split {significand}, {exponent}
				.Select(Double.Parse).ToArray();

			var number = numbers[0] * Math.Pow(10, numbers[1] % 3); // significand > 0 && < 1000
			var i = (numbers[1] / 3) - 1; // index to select in chars
			if (i >= symbols.Length)
				throw new ArgumentOutOfRangeException("bigNumber"); // 1.0E+27 is the max. Check before to optimize

			var letter = i >= 0 ? symbols[(int)i].ToString() : String.Empty; // get valid letter
			return number + letter;
		}

		private static double DehumanizeBigNumber(string humanizeBigNumber)
		{
			const string symbols = "kMGTPEZY";
			var letter = Char.IsLetter(humanizeBigNumber.Last()) ? humanizeBigNumber.Last().ToString() : String.Empty;

			return String.IsNullOrEmpty(letter)
				? Double.Parse(humanizeBigNumber)
				: (Double.Parse(humanizeBigNumber.Remove(humanizeBigNumber.Length - 1))
				  * Math.Pow(10, 3 + symbols.IndexOf(letter, StringComparison.Ordinal) * 3));
		}
}

.NETFiddle